Google Instant searches "as you type, not after you type," Marissa Mayer, Google's vice president of user experience, said at a press event at San Francisco's Museum of Modern Art.

The new tools are being rolled out to Google users in the U.S. today, and will spread internationally through the week.

Technology Live: Mashable's liveblog of the Google announcement.

Google's new search feature tries to figure out what a user is searching for as each letter enters the query box. For example, typing the letter "w" causes Google to speculate that a user is looking for the weather. It instantly displays a local forcast.

Similarly, typing "the girl" will display instant results to the popular book The Girl With The Dragon Tattoo. There's no need to type the entire title and press enter.

Mayer said the average Google query takes 15 seconds to enter and 300 milliseconds to process. Google Instant is designed to cut down on that time.

"We estimate this will help users save two to five seconds per query...or 11 hours for every passing second," she said.

The new feature "makes search more interactive. Power users will really appreciate it," says Greg Sterling, an analyst with researcher Sterling Market Intelligence.

Google is developing a version of instant search for mobile devices, such as cellphones. That may have the biggest impact, "because of the fewer keystrokes," Sterling says. "It will make mobile search more widely used."

Google Instant could be a blow to rivals because it offers a "much faster experience," says Danny Sullivan, editor of Search Engine Land. If they search using Microsoft's Bing, "it will seem so much slower, like they're using their grandfather's search engine," he says.

Websites will also need to adapt. Businesses who optimize their pages so that they appear near the top of Google results will have to "pay more attention to the suggestions offered by Google" that now pop up instantly as you type, Sullivan says.

Google also said it passed the a major milestone recently — it now has one billion users per week.